1. Field of the Invention
The invention relates to an apparatus and method for in vivo ultrasonic angioplasty. The apparatus of the invention may also be employed in an ultrasonic imaging system wherein it is used to generate acoustical contrast medium in situ.
2. Related Art
Arterial occlusions formed by thrombi and/or plaque deposits pose a serious threat to health. These deposits can result in a decrease or total blockage of circulation and lead to such conditions as peripheral vascular disease, angina pectoris and heart attack.
There are various known surgical techniques which may be employed to eliminate such occlusions, including replacement of the affected section of artery. Much work has been done toward development of non-surgical techniques in order to reduce the concomitant risk and trauma to the patient.
For example, one of the first non-surgical techniques developed was the balloon catheter which can be advanced into the circulation to dilate narrowed arteries. Such balloon catheters are well adapted for percutaneous insertion into the patient. This treatment method is generally referred to as "percutaneous transluminal angioplasty".
However, the unpredictable problems of abrupt closure and late restenosis of the dilated segment continue to compromise the overall results obtained with percutaneous transluminal angioplasty. High restenosis rates after coronary angioplasty of approximately 33%, and in multivessel angioplasty of approximately 68%, diminish the overall value of this technique even when one considers the low morbidity associated with the procedure. In addition, morphological studies indicate that the clinical improvement resulting from percutaneous transluminal angioplasty is accompanied by only a small increase in the diameter of the occluded artery. The mechanism of successful angioplasty involves internal disruption in fracturing of the atheroschlerotic plaque with splits extending to the media an through it. Both splits and fractured plaques resulting from the angioplasty were later found to have been repaired by clots formed thereon. The relatively small channel reopened by percutaneous transluminal angioplasty combined with the injury caused to the arterial wall may account for the high re-occlusion rate. The high rates of early and late re-occlusion after peripheral and coronary angioplasty thus appear to be independent of the operator's skill and the quality of equipment but, rather, inherent in the procedure itself. There is accordingly great interest in either improving or finding alternatives to balloon-based systems and procedures.
The alternative to balloon angioplasty which has been most intensively researched to date--the laser-based angioplasty systems--offer the apparent ability to open a cleaner, wider channel by evaporation of plaque and thrombi. Laser excision of pathological tissue is, however, limited by the operator's ability to precisely control the depth of ablation and limit thermal injury to the target tissue. To date, the use of lasers in this manner has remained largely experimental, with the high rate of arterial perforation being the major practical limitation.
The concept of using acoustic energy for vascular intervention has been known for over twenty years. Early researchers noted that ultrasound could destroy atheroschlerotic plaque and thrombi while leaving the underlying healthy vascular tissue undamaged. Experience with ultrasonic scalpel surgery has demonstrated that healthy vascular tissue is particularly resistant to ultrasonic energy. Recently, attention has once again been focused on the potential of ultrasound in vascular intervention. However, two problems have heretofore hindered the development of practical ultrasound systems for percutaneous insertion. First, since the ultrasound generator must be located outside of the body, it is often necessary to transmit the ultrasonic acoustic energy over a relatively long distance of 25 to 50 centimeters or more in order to pinpoint this energy at the site of the arterial occlusion. Attenuation of the acoustic energy along the length of the transmission member thus results in a loss of efficiency for the system, reducing the energy that reaches the internal arterial site. This requires the delivery of greater amounts of acoustical energy by the ultrasonic generator which rapidly increases fatigue of the transmission member.
A second problem is that this attenuation of acoustical energy is manifested as heat. Thus, the transmission member--which is primarily disposed within the circulatory system of the patient during treatment--can heat up rapidly during operation. Such heating can have serious adverse effects on the patient--a rise in the temperature of the transmission member of as little as 10.degree. C., or less, can have serious deleterious effects. This limitation severely restricts the duration of time during which acoustical energy can be applied and also limits the amount of power which can safely be applied to the transmission member by the ultrasound generator.
Still another problem inherent in the use of any percutaneous technique is the ability to accurately position the tool, whether it be a balloon, a laser or an ultrasound transmission member, at the site of the occlusion.
U.S. Pat. No. 3,352,303 of Delaney teaches a method for blood clot lysis using a probe-catheter apparatus which generates vibrational wave energy at its tip. According to this patent, blood clots may be lysed by direct application of acoustical energy for short periods of time. However, a disadvantage of this method is that the time duration of application must be so short that the heating effects normally associated with the application of concentrated wave energy to the human body do not present a significant problem. Time durations of from 0.5 to 5 seconds are described. The probe or transmission member is constructed of either stainless steel or monel metal.
The apparatus according to U.S. Pat. No. 3,352,303 may also include optional means for introducing a radiopaque fluid via the catheter to locate the site of the thrombis and to position the catheter in relation thereto. Additionally, this apparatus may incorporate a further optional cooling fluid in the catheter for cooling the probe and, according to the patent disclosure, reducing losses in acoustical energy along the length of the probe.
U.S. Pat. No. 3,565,062 of Kuris describes an ultrasonic system for removing accumulations of cholesterol-bearing and other deposits from the circulatory system. In this patented system, ultrasonic energy is transmitted via a catheterized ultrasound transmission member to the site of the deposit. No specific materials of construction are disclosed for the transmission member. However, it is noted by the patentee that the transmission member will have a series of nodes or antinodes resulting during ultrasonic vibration. For prolonged of use, substantial heat is generated at the antinodes--periods so much that a red glow is visible at spaced apart locations. The patentee equates this heating with the loss in acoustical efficiency.
One way of overcoming such noticeable heating, according to the Kuris patent, is to continuously vary the ultrasonic frequency to shift the position of the nodes and antinodes. This procedure, however, does not overcome the problem of acoustical energy loss in the transmission member but, rather, merely serves to prevent the occurrence of localized overheating by spreading out the heat losses over the length of the member.